Trigger Effects (誘発効果, Yūhatsukōka) activate whenever a certain condition on the Field, the nature of play, or the progress of the turn, is met.
Trigger effects are considered to be Spell Speed 1, but that doesn't mean you can activate them when ever you want, only when the condition has been met.
Optional Trigger Effects are effects that don't happen automatically when the trigger condition is fulfilled, the controlling player decides whether to activate it or not. These effects involve cards like "Mystic Tomato", "Peten the Dark Clown", or "Dark Magician of Chaos". With "When... you can..." Optional Trigger Effects it is possible to Miss the Timing.
When two Optional Trigger Effects try to activate at the same time such as two copies of "Mystic Tomato" destroy each other in battle, neither will miss the timing, as the last thing to happen was both being sent to the Graveyard. Both of the effects trigger at the same time, and form a Chain. The Turn Player's effect becomes Chain Link 1, and the opponent's effect becomes Chain Link 2.
Only the activation of an Optional Trigger Effect can miss the timing. In other words, as long as the effect is activated, the resolution will never miss the timing. So two monsters will be summoned in the above example.
Compulsory Trigger Effects are effects that must resolve, and are not Optional. When SEGOC, and two or more effects trigger at the same time, the Compulsory effects all go to the beginning of the Chain, and the Optional effects are put to the end of the Chain.
List of TCG and OCG Monsters with Trigger Effects
|Japanese name||Rank||Level||Attribute||Type||Card type||ATK||DEF|
|8-Claws Scorpion||八つ手サソリ||2||DARK||Insect||Effect Monster||300||200|
|Absolute Crusader||レアル・クルセイダー||4||EARTH||Warrior||Effect Monster||1,800||1,200|
|Absorbing Kid from the Sky||吸収天児||4||LIGHT||Fairy||Effect Monster||1,300||1,000|
|Achacha Archer||アチャチャアーチャー||3||FIRE||Warrior||Effect Monster||1,200||600|
|Adhesive Explosive||瞬着ボマー||3||WIND||Machine||Effect Monster||1,000||1,000|
|Air Armor Ninja||機甲忍者エアー||4||WIND||Warrior||Effect Monster||1,400||1,400|
|Airknight Parshath||天空騎士パーシアス||5||LIGHT||Fairy||Effect Monster||1,900||1,400|
|Alien Ammonite||エーリアンモナイト||1||LIGHT||Reptile||Effect Monster||500||200|
|… further results|